1. Field of the Invention
The invention relates to seat reclining apparatus and, more particularly to seat reclining apparatus for vehicles.
2. Description of the Prior Art
A reclining seat for vehicles, as shown in FIGS. 1 and 2, includes a seat cushion 1 and a seat back 2. The seat cushion 1 is fixed to a lower bracket 5 and the seat back 2 is fixed to on an upper bracket 9 rotatably connected to the lower bracket 5. According to the operation of a lever 3, the seat back 2 is pivotable and the reclined position of the seat back 2 is adjustable.
A conventional seat reclining apparatus 4, which folds the seat back 2 or changes the reclined position of the seat back 2, is shown in FIGS. 2 to 4. Three pins 6,7 and 8 are fixed on the lower bracket 5 so that three segmnts among the pins 6,7 and 8 define a triangle. The upper bracket 9 is rotatably mounted on the hinge pin or first pin 6 and a ratchet 10 having a pitch radius (R1) around the first pin 6 is fixed on the lower side of the upper bracket 9. A pawl 11 engageable with the ratchet 10 is rotatably supported around the pin 7. The lever 3 is rotatably mounted on the third pin 8 and is biased in counterclockwise directon as viewed in FIG. 2 by a spring 12. A cam groove 13 is formed in the lever 3 and a cam pin 14 fixed on the pawl 11 is slidably engaged therewith. A cam piece 15 is fixed on the lever 3 so as to abut against the pawl 11 for maintaining the engagement between the ratchet 10 and the pawl 11.
By raising the lever 3 from the engaged position shown in FIG. 2, the lever 3 rotates about the third pin 8 in clockwise direction and the cam piece 15 also moves about the third pin 8 in the same direction. Thus, the position of the cam pin 14 fixed on the pawl 11 is changed in the cam groove 13. Since the cam groove 13 is formed in a bent shape, the pin 14 slightly moves downwardly upon the clockwise rotation of the third pin 8. This result means that the pawl 11 is rotated around the pin 7 and released from the engagement with the ratchet 10.
In the released condition, the upper bracket 9 is rotatable around the hinge pin 6 as shown in FIG. 3. By fully rotatating the upper bracket 9 towards the direction as shown with an arrow (B), the seat back 2 is folded. Also, by releasing the lever 3 after rotating the upper bracket 9 towards one of the directions as shown with arrows (A) and (B), the pawl 11 is again engaged with the ratchet 10 at another position thereof. And as the pawl 11 is abutted by the cam piece 15, the engagement between the pawl 11 and the ratchet 10 is maintained. Namely, the reclined position of the seat back 2 is changed.
In the conventional seat reclining apparatus, as shown in FIG. 3, the upper bracket 9 rotates between the fully reclined position indicated by the solid line and the forward most position indicated by the dashed line. In accordance with the above rotation of the upper bracket 9, the ratchet 10 mounted on the upper bracket 9 is also rotated. The range of the teeth portion of the ratchet 10 corresponds to the rotating range of the upper bracket 9. Thus, a range for housing the ratchet 10 and permitting the rotation of the ratchet 10 will be approximately twice as much as that of the rotation of the upper bracket 9.
While the conventional seat reclining apparatus offers relatively wide range for operation the ratchet 10, the pitch radius (R1) in FIG. 2 of the ratchet 10 should be minimized so as to be housed in the lower bracket 5. This is necessary in case a large load is placed on the upper bracket 9, because the load operating the engagement portion of the ratchet 10 and the pawl 11 grows so large that the teeth of the ratchet 10 and the pawl 11 may be broken.
To avoid the breakage of the teeth of the ratchet 10 and the pawl 11, the size of the teeth of the ratchet 10 and the pawl should be considered. However, the adjustable step, which is determined by the size of the teeth, is also increased and it is impossible to finely adjust the reclining positions of the upper bracket 9. Further, since the pitch radius (R1) of the ratchet 10 is small, by the play of the pawl 11 around the pin 7 or the play of the upper bracket 9 around the pin 6, the upper bracket 9 will also rattle in the large range at the top end thereof. As shown in FIG. 4, the teeth of the pawl 11 is formed so as to prevent the both end teeth (a) and (b) from interfering with the teeth of the ratchet 10. Further, many teeth have to be provided on the pawl 11 so as to give enough strength to the engagement between pawl 11 and ratchet 10. Thus, the pressure angle (.alpha.) of the pawl 11 will become relatively large.
Accordingly, the large outward force (P1) is given by the force (F) operating the upper bracket 9 through the pressure angle (.alpha.1). Since the large outward force (P1) is exerted upon the cam piece 15, the cam piece 15 and the lever 3 respectively have to have enough thickness to accomodate the increase in the mechanical strength of the pawl 11 and the ratchet 10. This will cause the weight and cost of the cam 15 and lever 3 to be increased considerably. Also, since the cam 15 is moved donwardly by receiving such large outward force (P1), the engagement between the pawl 11 and the ratchet 10 may loosen and it becomes impossible to surely maintain the upper bracket 9 at the locking state.